In fiberoptic communication terminology, couplers are passive devices that connect three or more optical waveguide ends to split signals from one or more inputs to one or more outputs. Couplers may be made using fiber or planar waveguide technology. Combiners are devices by which the light signals from more than one input fiber or waveguide are combined into a single output fiber or waveguide. The term “coupler” will be used hereinafter to denote both types of devices.
The present invention is primarily adaptable to fiber couplers and combiners, and to fused biconic tapered couplers, i.e. couples of the type wherein two or more fibers are fused together and elongated, without the use of an outer protective glass tube.
It is known to arrange optical couplers or combiners into a cascaded array. Such array is illustrated and described e.g. in U.S. Pat. No. 6,023,542 issued Feb. 8, 2000 and assigned to the assignee of the present invention.
In order to arrange two or more couplers in a sequence, two or more couplers are prepared separately and are then spliced. This process results in an array, or a cascade, of couplers, that has a relatively large dimension caused by the “pigtails”, or lengths of optical fibers that are an integral part of a coupler after manufacturing. Conventionally, one tries to isolate fibers not used in forming a particular coupler, away from the heat source used to fuse that coupler. The fibers are usually bent away from the heat source. This imposes a certain length for the pigtails and a resulting distance between two sequential couplers.
It is desirable to reduce the dimension of an array or cascade of couplers and eliminate the loss concomitant with splicing. It is also desirable to provide a monolithic device consisting of more than one fused biconic coupler on one substrate.
Fiber couplers may form part of Mach-Zehnder type interferometers wherein two fibers are fused into a coupler at one end and at the other end of the interferometer. The two couplers are thus connected by two fibers. The two ends may be spaced by only a small distance, say in the order of a few centimetres or even less. Another known possibility is a tree (1×N) coupler or a star coupler where more than two fibers are fused into a coupler. These couplers can of course be spliced to form a cascade or array of couplers.